1. Field of the Invention
The present invention relates to a thermal transfer sheet, and in particular to a thermal transfer sheet to be used in order to form an image in a receptor layer of an intermediate transfer recording medium which is one kind of receptor layer-transfer sheets utilized to transfer the receptor layer on a transfer receiving material together with the image previously formed in the receptor layer.
2. Description of the Related Art
Heretofore, there have been known various types of thermal transfer recording methods, in which a thermal transfer sheet, having a substrate sheet on which a coloring material layer is disposed, and a transfer receiving material, on which a receptor layer is disposed, are mutually superposed and pressed between a heating device such as a thermal head and a platen roll, and the heat-generating members of the heating device are selectively heated depending on image information. As a result, the coloring material contained in the coloring material layer of the thermal transfer sheet migrates to the transfer receiving material, realizing the thermal transfer recording. Of these methods, a heat fusion type thermal transfer method and a sublimation type thermal transfer method have been most commonly used.
In the heat fusion type thermal transfer method, a thermal transfer sheet carrying a heat fusible ink layer is heated by the heating device as mentioned above, and softened or molten heat fusible ink is transferred to the transfer receiving material such as a paper made of natural fiber, plastic sheet or the like, thus forming an image. The heat fusible ink layer is composed by dispersing a coloring material such as pigment into heat fusible binder such as wax or resin, and carried on a substrate sheet of the thermal transfer sheet such as plastic film. Images formed by this heat fusion type thermal transfer method have higher density and superior sharpness, and this method is therefore suitable for recording of binary images consisting letters and/or drawings. In addition, it is possible to form a multi-color image by subjecting the transfer receiving material to a transfer process with the use of a thermal transfer sheet having heat fusible ink layers of various colors, for example, three colors comprising yellow, magenta and cyan or four colors further comprising black.
In the sublimation type thermal transfer method as another common method, a thermal transfer sheet carrying a sublimation dye layer is heated by the similar heating device, and the dye is sublimated from the layer and transferred to the transfer receiving material, thus forming an image. The sublimation dye layer is composed by dissolving or dispersing the sublimation dye of the coloring material into a binder resin, and carried on a substrate sheet of the thermal transfer sheet such as plastic film. The sublimation type thermal transfer method is capable of controlling an amount of the transferred dye by each dot depending on extent of energy applied from the heating device such as the thermal head, thus realizing reproduction of gradation via change of density. In addition, since the dye is utilized as the coloring material, the formed image has an improved transparency, thus providing an improved reproducibility of an intermediate color prepared by superposing plural colors with the use of the corresponding dye layers. Accordingly, it is possible to form a full color or natural color image having a high quality, when the dyes of three or four colors are transferred to the transfer receiving material with the respective colors superposed by using the thermal transfer sheet having the plural sublimation dye layers of three colors comprising yellow, magenta and cyan or four colors further comprising black.
When a color image is formed by the thermal transfer method, additional use of the black ink improves sharpness of the image such as letters or the like transferred to the transfer receiving material. The heat fusible black ink has a further feature that a single color image such as letters, symbol or the like formed by thermally transferring the heat fusible black ink is readable by an optical character reader, and it is particularly utilized to print a bar code or the like. In contrast with this, an image formed by the sublimation type thermal transfer method is inferior in readability on the optical character reader, because infrared absorption of the image is relatively small at about 900 nm. Accordingly, when optical character recognition on a machine is required, the heat fusible black ink is utilized in order to form a single color image of black or a multi-color image comprising black and another color even though most of image is formed by the sublimation type thermal transfer method.
Among the above mentioned thermal transfer methods, transferring performance of the sublimation type thermal transfer method is strictly influenced by dyeability of the transfer receiving material to be provided with the image. Therefore in a case where a surface of the transfer receiving material is short in the dyeability, it is almost impossible to form the image insofar as a receptor layer having the dyeability is formed on the transfer receiving material.
In order to solve the problem mentioned above, Japanese Patent Application Laid-Open (JP-A) Nos. 62-264994 discloses a method in which a receptor layer is peelably formed on a substrate film to prepare a receptor layer-transfer sheet, and the thus formed receptor layer of the receptor layer transfer sheet is transferred onto a transfer receiving material having a poor dyebility, and then a dye contained in a dye layer of a thermal transfer sheet is transferred to the receptor layer of the transfer receiving material to form an image. It is possible to form the image on the transfer receiving material having a poor dyebility according to this method. Japanese Patent Application Laid-Open (JP-A) Nos. 3-45390 discloses a thermal transfer sheet applicable to this method, on which respective sublimation dye layers of yellow, magenta and cyan and a heat fusible black ink layer are alternately arranged side by side with a peelable layer interposing under the heat fusible black ink layer. There is also proposed a peelable layer having a improved durability so as to serve as not only the peelable layer as it is but also a protect layer.
Japanese Patent Application Laid-Open (JP-A) Nos. 62-238791 discloses another method in which a receptor layer is peelably formed on a substrate film to prepare a receptor layer-transfer sheet, and a dye is transferred from a thermal transfer sheet to the thus formed receptor layer of the receptor layer-transfer sheet to previously form an image, and thereafter the receptor layer provided with the image is transferred from the receptor layer-transfer sheet onto a transfer receiving material. According to this method, it is also possible, whether the transfer receiving material has a good dyeability or not, to form the image in the transfer receiving material even if the transfer receiving material has a surface difficult for operating of the heating device, for example, one having a curved surface such as a tumbler, another one of a fixed structure such as a wall and still another one liable to be thermally fused by heating of the thermal head or the like. A receptor layer-transfer sheet applicable to this method is called as an intermediate transfer recording medium. The above mentioned thermal transfer sheet on which the sublimation dye layers of the respective colors and the heat fusible black ink layer are alternately arranged side by side with the peelable layer interposing under the heat fusible black ink layer is similarly used to previously form the image in the intermediate transfer recording medium.
However, if an image is formed in the receptor layer of the intermediate transfer recording medium by transferring the heat fusible black ink layer of the thermal transfer sheet, The transferred heat fusible black ink layer is accompanied by the peelable layer having a poor adhesive property with respect to the transfer receiving material, thereby forming an uppermost layer having a poor adhesive property. Therefore, when the receptor layer provided with the thus formed image is transferred from the intermediate transfer recording medium to the transfer receiving material, adhesion between the image and the transfer receiving material is partially deteriorated. That is, there is caused a defect that a portion of the receptor layer provided with the image formed of the heat fusible black ink layer is hardly bonded to the surface of the transfer receiving material, and loosened therefrom.